Metroids are organisms created by the Chozo. First discovered on the planet SR388 years after their introduction, they were promptly used as a bioweapon by the Space Pirates. Metroids feed on the "life energy" of their prey in a manner similar to the way a leech drains a host organism's blood. Unlike leeches, Metroids leave all of their prey's blood and internal organs intact yet lacking the energy necessary to sustain life.[1] It is unknown exactly how the Metroid extracts this "life energy" or what, in fact, said life energy consists of.

The Metroids were long rumored to be created by an ancient race. It was eventually revealed that the Chozo were the ones who genetically engineered the Metroids in order to prevent the spread of the more dangerous X Parasites. Interestingly, the word "Metroid" in the Chozo language roughly translates to "Ultimate Warrior".

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The body of a "typical" Metroid consists of a thick gelatinous membrane (resembling that of a jellyfish) and contains a set of three (in Phazon Metroids) to four (in Tallon Metroids) red, raspberry-shaped, quadripartite/tripartite nuclei. They have two pairs of mandibles, one for gripping their prey and the other for extracting its life energy. Metroids appear capable of directly sensing the life energy of their prey, as they have no visible sensory organs. In the early stages of their development, Metroids hover in the air. Metroid Prime 2: Echoes explains that they defy gravity by using kinetic energy that is stored in their bodies. These mature Larval Metroids are the ones most often encountered in the Metroid games and are in the third natural stage of their life-cycle, preceded by the Infant Metroid and Egg stages. Traditionally, Larval Metroids are transparent or translucent with hints of certain colors (green, red, blue, etc.).

A Metroid dividing due to heavy exposure to beta radiation.

Metroids are vicious creatures with no natural predators, being at the top of the food chain on SR388. They appear to be quite intelligent - for example, when a Metroid is released into a room containing both Space Pirates and Samus Aran, it will attack Samus Aran first, her being the most lethal enemy. Once attached to their prey, Metroids are nearly impossible to dislodge and will quickly drain their victim's life force completely, often killing it in seconds. Using Morph Ball Bombs is the only known way of dislodging an attached Metroid; consequently, Samus is one of the only two beings capable of surviving Metroid attacks, the other survivor being Mother Brain by using an unknown 'resurrection' ability.

Metroids have an endless appetite and will feed on any and all living creatures they come across with the exception of their own species. This immense threat is compounded not only by their invulnerability to most forms of weaponry, but also because exposure to extremely high amounts of beta radiation causes them to asexually reproduce via division much like a cell undergoing mitosis; the latter ability is seemingly restricted to the Infant and Larval stages of a Metroid. Metroids have only one real weakness: an extreme sensitivity to cold temperatures. Once their membrane is frozen, it can be shattered by concussive weaponry (such as Missiles and Super Missiles), killing the Metroid. Power Bombs can also harm them, but their level of effectiveness on Metroids vary immensely. In Corruption, Samus was able to use the X-Ray Visor to lock onto a Metroid's nuclei, allowing her to use the high-frequency Nova Beam to penetrate the protective membrane and strike the nuclei directly, instantly dispatching the creature. While most Metroid variants seen in the Prime games still retain their weakness to cold, some specific mutated strains have rid themselves of this vulnerability.

In their natural life cycle (as seen in Metroid II: Return of Samus and near the end of Metroid Fusion), Metroids gradually metamorphose into a creature resembling an insect or a crustacean before finally attaining a somewhat reptilian form. As they change, they eventually replace their gelatinous membrane with a hard organic shell and grow a head along with a set of limbs. The shell removes their weakness to cold temperatures, but all forms retain a section of weak, uncovered membrane containing a single nucleus which is vulnerable to attack. The only known form with a complete shell is the mutated Metroid Prime's first form. The egg-laying Queen Metroid appears to be the only form in which Metroids can naturally reproduce. As they grow, Metroids also gain the ability to project bolts of static electricity or spit toxic saliva at their prey.

Space Pirate Data states that Metroids mutate differently depending on what planet they are exposed to,[2] such as the Hunter Metroids encountered on Tallon IV in Metroid Prime. Metroids cannot mutate into their natural advanced stages seen in Metroid II and Fusion unless they are on SR388 or on a perfect replica of the planet. When mutated by Phazon, Metroids will also gain various odd qualities such as the ability to phase through walls.

Metroid Eggs on the Pirate homeworld.

The Metroids live on a mysterious energy that they drain from their victims. What this energy is remains unknown; the victim loses no bodily fluids but perishes nonetheless. As of Metroid Prime, this energy is referred to as "life energy". Metroids are stated to be energy-based organisms and thus increase in mass as they drain energy. Metroids can also feed off the energy used by Samus's Power Suit, which will protect Samus herself as long as it is active. However, were her suit to lose all of its energy, Samus would be rendered vulnerable to the Metroids. This almost happens to her in Other M; as she is about to kill a Baby Metroid she encounters, Adam Malkovich fires at her with a Freeze Gun. The shot, combined with her emotional duress at the time (the Metroid here reminded her of her baby) causes her to lose her Power Suit. Adam shoots the Metroid and freezes it before it can attack Samus.

In addition to draining life energy from victims, Metroids can heal other life forms by transferring stored life energy into them as demonstrated by the baby in Super Metroid. This stored energy can also be drained from the Metroid, allowing it to be used as a living rechargeable power cell. However, a scan of a Tallon Metroid in Metroid Prime 2: Echoes reveals that repeated energy drains can cause cellular breakdown to occur in Metroids.

Metroids! This species of energy draining, unicellular aliens can be frozen, then shattered with a blast from one of my missiles, but when they are in their normal, pulsating state, they can absorb my attack. They're treacherous!

"This protoplasm in suspended animation was discovered on the planet SR388. It clings onto Samus' body and sucks his[3] energy. It can't be destroyed directly with the normal beam. Freeze it with the ice beam, and then fire 5 missile blasts at it."

FLOATING LIFE FORMS - METROIDS"It is said that Metroids are life forms created by an ancient civilization. Metroids engulf living creatures and absorb their energy. They are very intelligent and quick to reproduce."

"SPECIMEN ID# M-1A"Extremely dangerous! Gelatinous exterior of Metroid makes it impervious to beam weapons. Metroids can only be destroyed by Missiles while frozen. Large claws grip prey while smaller front fangs drain its life force. Transform to Morph Ball and use Bombs to escape grip. DANGER LEVEL: HIGH"

"Metroids are energy-based parasitic predators. The dominant species of planet SR388, Metroids can suck the life force out of living things. A Metroid latches onto its prey and drain energy, growing larger as it does. The only way to shake an attached Metroid is to enter Morph Ball mode and lay a Bomb."

"Metroids are highly dangerous parasites averse to cold temperatures. The dominant species of planet SR388, Metroids can suck the life force out of living things. A Metroid will latch onto its prey and drain energy, growing larger as it does.

"What's the matter? All I said was that Komaytos look like little Metr—" Non-canon warning: This article or section contains information that may not be considered an official part of the Metroid series in the overall storyline by Nintendo.

Metroids appear in the Kid Icarus franchise, under the name Komayto, which is stated in the instruction manual to have come from another planet. This creature can be found flying around Skyworld in packs. Kid Icarus was also created using the Metroid game engine. During the game Kid Icarus: Uprising, the main character Pit states himself that "Komaytos kind of look like little Metroids" before being hushed by Viridi.

A Metroid is a collectible treasure in Wario Land II. They are also often in 9-Volt's microgames in the WarioWare series, such as WarioWare, Inc.: Mega Microgame$!, WarioWare, Inc.: Mega Party Game$!, WarioWare: Touched! and WarioWare: Twisted!.

Metroids made an appearance in Kirby's Dream Land 3 for the SNES. If Kirby freezes all the Metroids in the level, Samus removes her helmet and gives Kirby a Heart Star.

In Metroid Prime 3: Corruption, a Metroid is used on the tab for the creatures section of the Logbook. Interesting to note is that the Metroid depicted here has six nuclei, five encircling the top of a larger, centralized nuclei.

The Baby from Metroid II: Return of Samus and Super Metroid appears as an item in the Nintendo DS game Animal Crossing: Wild World. It appears in its signature capsule, and if tapped will play a few seconds of the Super Metroid title screen theme. It is a rare item that is acquired randomly by shooting down Gulliver. The item also appears in Animal Crossing: City Folk, but cannot be transferred through the ingame catalogue due to the item's rarity. It acts the same way as before and is obtained in the same manner as well; it will play the entire Super Metroid theme this time however. The same item appears once again in Animal Crossing: New Leaf, but it is not obtained through the same method as the two previous games. Instead, it is randomly acquired through fortune cookies, which the player can buy using two of the 3DS's Play Coins. This version also plays only a few seconds of the title theme.

In Tetris DS, there is a single player game mode called Catch Mode where players catch Tetraminos (blocks) in order to create a 4X4 square and detonating it to make the cluster smaller. The Brinstar theme plays during this and encapsulated Metroids (in capsules similar to the Metroid hatchling's) fall down. If touched, the energy meter will go down and the cluster becomes smaller.

Metroids appear in a minor reference in Super Smash Bros., in Super Smash Bros. Melee as a trophy as well as the Baby being seen in the opening cutscene, and in Super Smash Bros. Brawl and Super Smash Bros. for Nintendo 3DS and Wii U as an Assist Trophy that will latch onto opponents and "drain" them of energy (what they actually do is increase the victim's damage, but can still sap health if a stamina match is being played). It can be shaken off, which will cause it to try and seek another victim. It can phase through platforms similar to a Phazon Metroid. Using Up-Special moves while the Metroid is latched on to a character cancels the move after the few starting frames; this can be done multiple times in a row. This allows some characters to stay in the air for quite a while. In Melee, the intro scene from Super Metroid is reflected off of the trophy, with the infant missing from the scene.

"A parasitic life form that can absorb all types of energy, Metroids have strong resistance to most conventional weaponry. To defeat them, Samus Aran had to freeze them with Ice Beam shots and then blast them with missiles. Mochtroids, which look like Metroids, are weaker, with fewer internal nuclei."

"A bioengineered life-form found on planet SR388. Metroids attach to organisms and drain life energy. The Galactic Federation commissions Samus to eliminate them, but Space Pirates try to harness their power. One of the few ways Samus can kill Metroids is by shooting them with the Ice Beam and then shattering them with missiles. The Metroid's cry is chilling and indescribable."

"First discovered on planet SR388, this floating life-form grows by absorbing the energy around it. It is incredibly durable but extremely weak to cold. In Smash Bros., a Metroid will grab a rival by the head and drain energy. If you get grabbed, shake it off by pressing left and right!"

PAL

"Mysterious floating life-forms, discovered on the planet SR388, that absorb energy from other living beings. They're tough, except when they're cold. In this game, they'll try to attach themselves to fighters' heads and drain their energy. Press all the buttons you can to shake them off!"

"No! Stop! Get off my head!"

Smash Bros. DOJO!!

A hidden Mr. Saturn sprite can be seen on the Metroid through hacking.

Metroids appear as robots. They can suck out the Mii's energy but they also like to drop Miis in the Lava on the volcano map. Metroids must be frozen with the Ice Charge before shooting their core to kill them, reminiscent of how they are killed in the official Metroid games. This game also implies that the Chozo went extinct because they lost control of the Metroids.

The unidentified energy that Metroids feed off of has never been fully disclosed in the games. It could be theoretically possible that the so-called life force drained by Metroids may be Mana, an incorporeal life energy believed to be possessed by all living beings. Given that the Chozo understood nature and spirituality through their advanced knowledge, it is possible that they genetically implemented such metaphysical attributes of energy draining into the Metroid's biology.

However when explaining on a Biological scale; the energy source may either be Adenosine Di/Triphosphate (ADP/ATP); transports chemical energy to cells for metabolic purposes or thermal energy taken by catalyzing ADP/ATP reactions; which leads to one way the subject dies is by raising the body temperature of the host by a significant degree so that life cannot be sustained.

Chronologically, Samus' first encounter with Metroids occurred in Tourian; ironically, the last of the species would later become extinct there.

The eggs which can be found on the Pirate Homeworld in the room Metroid Processing are in a tank with large metal 'hands' clasping them. This is similar to the scene in Jurassic Park, where dinosaur eggs are being produced and hatched with a similar metal claw.

Bioforms that are killed by Metroids turn into husks with characteristics that, for unknown reasons, vary during time. Some turn into an extremely fragile brown husk that crumbles into dust on contact, while others become hardened corpses that cannot crumble. In Zero Mission, victims acquire a very darkened color.

It may be possible that the cause for biological breakdown of victims could be that Metroids have a similar effect to fleas or other parasites—they might contain some sort of venom that breaks down the bioform over time.

Metroids have never been successfully contained without them somehow breaking free, either through their own means or through the help of another being, intentional or not (these include Samus Aran, Dark Samus, Madeline Bergman and SA-X).

Additionally, it is inconclusive if Mother Brain's powerful brainwaves have maintained complete dominance over Metroids, as a few varied incidents in the past have provided some skepticism to the level of control Mother Brain has had on the Metroids, despite Samus stating that Mother Brain's telepathy is needed in order to control Metroids.

There have been no known cases of intraspecific competition between Metroids for right to feed. In occasions where several Metroids are in the presence of a potential prey, they will take turns attacking the victim (in the Prime series and Zero Mission gameplay), or will group together to feed on a victim simultaneously (as seen in Metroid 2, Super Metroid and during a cutscene in Zero Mission).

Dark Tallon Metroids were seen putting weakened Pirate Troopers in a large pile made up of the latter on Dark Aether, seemingly conserving their food for later group feedings. However, this behavior is most likely non-existent in other Metroid variants, as these Dark Tallon Metroids are actually controlled by the sentient race known as the Ing. Interestingly, this also means that the Ing were capable of overcoming their possessed victims' insatiable hunger and no longer devour their prey quickly and completely.

Strangely, although Infant Tallon Metroids are said to be used as batteries, a scan in the Metroid Processing room on the Pirate Homeworld (next to the controversial Metroid project 'Dread' scan) says that a project to use Metroids to power mobile weapons failed. It could be possible that this project was for the use of adult Metroids, instead of the weaker and more manageable Infants.

The full limitations of Metroid behavior has not fully been explored. However, it possibly extends beyond the simple feeding instinct, as demonstrated by the infant Metroid in Super Metroid. Technically, it had grown beyond the imprinting of Samus as its mother, yet it still recognized her and restrained itself, even going so far as to save her from Mother Brain and transfer energy and a new weapon back to her.

Ironically, the Galactic Federation has been successful in many elements concerning the Metroids, many of which the Space Pirates ultimately failed to achieve despite years of trial and error. The former organization had found an extremely reliable way of controlling Metroids in the form of the humanoid MB. They had also successfully rid the creatures of their greatest vulnerability: cold temperatures. Finally, the Federation was able to give the Metroids access to their natural mutations, despite the creatures being located outside of their homeworld, SR388.

The Metroids have actually been able to nullify their weakness to cold on one occasion, long before the Federation did. This was seen in specific Fission Metroid variants and Metroid Prime's core form during Samus's mission on Tallon IV. However, this invulnerability was unintentional, as these two Metroid species had achieved this ability through an unpredictable series of mutations caused by Phazon.

Metroids seem to not have the need to breathe oxygen. This is seen when entering the G.F.S. Valhalla; while exploring the destroyed ship, Metroids are seen outside of the ship and wandering about in space. Also, an Omega Metroid was in the hanger bay of the Biologic Space Laboratories research station, as the door was broken showing the vacuum of space, possibly having the room decompressing. This may shed light on one of the theories behind Metroid Prime's origin.

In Metroid Prime and Metroid Prime 2: Echoes, Metroids who have broken free from their containment tubes will engage in combat with the Pirates and vice versa. However, on the Pirate Homeworld in Metroid Prime 3: Corruption, both sides will ignore each other and assault Samus in tandem. A logical explanation would be that both sides, being corrupted by Phazon, no longer see each other as adversaries (this has been noted in Pirate research experiment reports found by Samus in Metroid Prime) or that both species are forced by Dark Samus to work as a group. This could also be due to a programming error, as both creatures never appear in the same room outside of containment unless Samus trips the alarm and enters a Metroid-infested room, forcing the Pirates to appear in it.

In Metroid Prime and Metroid Prime 2: Echoes, when a Metroid collides with Phazon, it will explode. This can also happen with any open Door, but this does not work with the Metroids in the Trilogy version.

In Metroid Fusion, Samus is saved from X infection by a Metroid vaccine only made from Metroid cells. This suggests that the Chozo engineered even the Metroid's base cells to prey upon those of the X.

Ironically, the Cold X of Metroid Fusion attempted to counteract against Samus due to her Metroid cells and her weakness to cold. It is doubtful Cold X existed back on SR388 due to the planet's lack of a cold climate. If possible, the Cold X could have slowed down the Metroid's activity on SR388, though it is unlikely they would have been able to kill a single Metroid, as they lacked concussive weaponry to shatter them. On the other hand, they could have used a large, powerful creature to break them (such as Arachnus).

In early encounters with the SA-X, the imitation would attempt to kill Samus by using the traditional tactic used to kill Larval Metroids (freezing her in place with the Ice Beam, then shooting a Super Missile at her). It is most likely from Samus's memories that the SA-X learned of this combination of attacks.

Some Space Pirates are known to have treated Metroids as pets. Although High Command strictly forbids this, some Metroids can be scanned with evidence of being recently fed with Space Pirate rations and pet treats. This caused the Metroids in question to suffer food poisoning, and occasionally became frenzied creatures.

Metroids are the Metroid series equivalent to the Alien series Xenomorph.

Coincidentally, the spinoff prequel film Prometheus, created well after the Metroid franchise, implies that the Xenomorphs were created by an advanced alien race for use as a bioweapon, similar to the Chozo developing the Metroids to stop the spread of the X Parasites.

In the real-time-strategy game Starcraft II: Wings of Liberty, on the level Piercing the Shroud it is possible to see a Tauren Space Marine and a Metroid.

In Metroid Prime 2: Echoes, several Metroids can be seen in the Space Pirate Agon base, apparently feeding on a possessed Space Pirate. They appear to be in a state of zero gravity, taking turns bumping the Space Pirate back and forth as the pirate is floating in the stasis tank. If the pirate is scanned, the scan states that the pirate is being leeched of energy. Strangely, the Metroids' life-sucking pincers do not touch the pirate, perhaps implying that the Metroids can leech energy just by coming into contact with their prey.

The Metroids seem to have various similarities to the Eratis Bug on Stargate Atlantis. The Eratis Bug latches onto a victim and begins draining it of its life energy. They are extremely hard to get off. Finally, their place on their homeworld is various caves throughout the planet. Metroids also share the life-draining attribute with the Wraith, who also appear in the same series as the Eratis Bug being an evolved version of said bug.

Note the four nuclei visible within the blue inset.

Although all species of Metroid seen in the actual Metroid games possess three spherical nuclei, Metroid: Other M concept art depicts a larval Zebesian Metroid with at least four nuclei.

The background of Metroid Prime Pinball 's Wireless Mission mode features a Metroid with many more, smaller nuclei and a jagged shell.

When reading the temporary scan of a creature in Corruption, a pair of Metroids are seen to either side of the scan text. The nuclei of the two Metroids are placed side to side and are connected by lines to the bottom of their membrane.

NES Remix includes a Metroid Miiverse stamp.

Phazon was originally explained as the remaining energy from the Metroids Samus exterminated on Zebes, undergoing fractal cloning.[3]

^While in the Metroid Processing area of Metroid Prime 3, when you scan the dissected Metroids you read "Observations on Metroids native to different planets display subtle variations in biological processes." such as in video [1]. When scanning a Metroid in a glass tube you read "Our Homeworld's unique atmosphere has resulted in various unique physiological mutations." such as in video [2].

^The English manual for Metroid deliberately referred to Samus as a male, not a female